Process for bonding rubber to polyester structures



April 1, 1969 H. T. PATTERSON 3,436,233

PROCESS FOR BONDING RUBBER T0 POLYESTER STRUCTURES Filed Jan. 5, 1965 FIG-1' SHAPED STRUCTURE OF LINEAR POLYESTER (PREFERABLY A FIBER CORD) COAT WITH EPOXYLATED NOVOLAK AND POLYAIIIDE IN ORGANIC SOLVENT CARRIER CURE COATING BY HEATINC I TO I50C.- 240C I COAT WITH RESORCINOL- FORNALDEHYDE AND SYNTHETIC RUBBER LATEX CURE COATINC I II APPLY RUBBER AND CURE REINFO RCED RUBBER PRODUCT CUSHION RUBBER BOTTON RUBBER INVENTOR HUGH T. PATTERSON United States Patent "ce 3,436,288

Patented Apr. 1, 1969 3 436 288 evaporate the solvent and cure at a temperature in the PROCESS FOR b RUBBER To range of 150 C. to 240 C. for a curing time of 45 sec- POLYESTER STRUCTURES onds to 5 minutes depending upon the temperature. Hugh T. Patterson, Greenville, N.C., assignor to E. I. Tl'lls P PIOVldeS Precoated Shaped stfucture du Pont de Nemours and Company, Wilmington, Del., a beaF1I 1g i g ofbthe feasctlon lg h( )f corporation of Delaware constltutmg rom a out 0. to a out 20 a by weig t 0 Filed Jan. 5, 1965, Ser. No. 423,528 the coated structure. The coating composition is applied I t- Cl- 2 7/ B3211 27/38;C09J to the polyester shaped structure by any conventional 156-142 5 Chums means including dipping, spraying, brushing, padding or 10 the like with the structure relaxed or under tension.

The precoated structure is then overcoated with an aqueous resorcinol-formaldehyde synthetic-rubber latex ad- ABSTRACT OF THE DISCLOSURE The process for bonding rubber to polyester cords comhesive mixture, which is dried and cured, whereupon the prising coating the cord with an organic solvent solution structure is ready for bonding to rubber in conventional containing a specific epoxylated novolak resin and a polyfashion. amide resin reaction product of polymeric fat acids having In the drawing, which illustrates specific embodiments at least two carboxyl groups and polyalkylene polyamines of the invention, having two primary amine groups and a plurality of sec- FIGURE 1 is a fiowsheet of the steps of applying and ondary amine groups, heating the coated cords to cure curing coatings on a fiber cord, or other shaped structure, the coating, overcoating the structure with a specific latex in the preparation of a reinforced rubber product, and mixture, drying and curing the overcoating, and then ap- FIGURE 2 is an isometric view of a portion of V-belt, plying and curing a layer of rubber on the coated structure. cross-sectioned at right-angles to the long axis of the belt,

to illustrate the structure produced as in Example III.

As will be seen by following the various steps outlined This invention relates to the treatment of fibrous main FIGURE 1, the polyester cord is coated with an orterial to improve its adhesion to rubber. More particularly ganic solvent solution of an epoxylated novolak and a it relates to a novel adhesive composition and method of polyamide resin, which coating is thereafter cured by heatapplication particularly suitable for synthetic linear coning to a temperature of 150 C. to 240 C. A second coatdensation polyester fibers. ing of resorcinol-formaldehyde synthetic-rubber latex mix- In contrast to the naturally occurring polymer fibers ture is applied and cured and then the rubber is applied such as cotton and the older synthetic fibers such as nylon, and cured to give a reinforced shaped product. the new polyester fibers have been found particularly The term epoxylated novolak resin is intended to difiicult to bond to rubber. The outstanding properties of refer to the reaction product obtained by treating a nonpolyethylene terephthalate fibers, and their commercial heat-hardenable phenol-formaldehyde condensation prodavailability, have made it highly desirable that a good uct with epichlorohydrin. Epoxylated novolaks and methpolyester-to-rubber adhesive be developed. A number of ods for their preparation are described by Hanson and such adhesives have been proposed, but none of these has Ringwald in British Patent No. 746,824 published Mar. been found fully satisfactory in certain critical applica- 21, 1956. Such novolaks are also described by Lee and tions. Neville in Epoxy Resins (McGraw-Hill, New York,

The present invention provides a novel adhesive system 1957), page 18. An idealized structural formula for an and process of bonding synthetic polyester materials to epoxylated novolak particularly suitable for the present rubber with good bond strength both at low and at high invention is:

' if LV temperatures. Furthermore, the invention is particularly where R may be a chlorohydrin, glycol, or polymeric suitable for the preparation of raw-edge V-belts reinether radical. Best results are obtained with an average forced with polyester cords where a coherent cord strucmolecular weight of 1100 to 1500, an oxirane oxygen conture highly resistant to fraying is required. The invention tent of 5% to 10% by weight, and an epoxide equivalent has the added advantage of providing adhesion to a wide of 210 to 260.

variety of rubber stocks. The term polyamide resins as used herein refers to In accordance with the present invention, a synthetic a class of resins prepared by reacting polymeric fat acids polyester shaped structure is pretreated wtih an organic having at least 2 carboxyl groups, with polyalkylene polysolvent solution containing dissolved therein 0.5% to 20%, amines containing two primary amine groups and a pluby weight of the solution, of (A) an epoxylated novolak raliy of secondary amine groups separated by alkylene resin having an average of at least 2 epoxy groups per groups. The polymeric fat acids are prepared by polymmolecule, an average molecular weight above about 540 erizing drying or semi-drying vegetable oils or the corand an epoxide equivalent within the range of 200 to 300, responding fatty acids of 12 to 22 carbon atoms, such and (B) a polyamide resin reaction product of dimerized as linoleic or oleic acids. Suitable types of vegetable oils vegetable oil unsaturated fatty acids and polyalkylene include soybean, linseed, tung, perilla, oiticica, cottonpolyamines, wherein the weight ratio of (A) to (B) falls seed, corn, tall, sunflower, safllower, and dehydrated castor in the range of /20 to 20/80, and preferably in the oil. Polymeric fat acids are typically dimerized mixtures range of 70/30 to 45/55. The liquid coating is heated to of acids which contain a smaller quantity of trimeric or higher polymeric acids and some residual unpolymerized acid. The preparation of polyamide resins is described by Floyd in Polyamide Resins (Reinhold Publishing Corporation, New York, 1958), page 25; by Lee and Neville in Epoxy Resins (McGraw-Hill, New York, 1957), page 166; and by Renfrew and Wittcolf in US. Patent No. 2,705,223 dated March 29, 1955.

The polyamide resins used in the present invention are liquid at room temperature and contain sufiicient free amine groups to have an amine value of at least 200 milligrams per gram of polyamide. Preferably the amine value is 290 to 320. The amine value is determined by titration with HQ and calculated as the milligrams of KOH equivalent per 1 gram of resin.

It is important that the novolak and polyamide resins be applied from solution in a volatile organic solvent. Any organic solvent for the two components, which is volatile and non-reactive under the conditions of application, can be used. Readily available organic solvents include acetone, chloroform, trichloroethylene, toluene, ethylene chloride, ethylene glycol monoethyl ether, and mixtures thereof such as toluene mixed with ethylene glycol monoethyl ether. Aqueous dispersions are not suitable. An organic solvent solution provides much higher levels of adhesion, and there is also much better penetration of cord structures to provide the highly coherent structure necessary for the successful preparation of acceptable raw-edged V-belts. By raw-edged V-belts is meant those V-belts which do not have a complete woven fabric covering. Such belts are usually made by building a wide belt and then cutting it into a plurality of narrower belts, with the result that longitudinal sections of the internal reinforcing cords are exposed at the cut surfaces.

The invention will be better understood from the examples, which illustrate some of the suitable embodiments, methods of carrying out the invention and advantages obtained. In these examples, the strength of the adhesive is determined by the single-end strip adhesion test (SESA). In preparing samples for the single-end strip adhesion test, lengths of treated cord are placed in the bottom of a steel mold, the cords being parallel with a spacing of one inch between cords. The cords are placed under dead-weight tension to maintain their position. A sheet of unvulcanized compounded elastomer stock, 125 mils in thickness, is placed over the cords, covered with a cotton duck reinforcing backing, and the top plate of the mold placed over the back. The mold is put into a platen press. A pressure of approximately 150 pounds per square inch is applied and the mold is usually heated to about 145 C. for 60 minutes. Other vulcanizing conditions appropriate for individual elastomer compositions may be used. Due to the flow of the rubber stock, the pressure within the mold falls to a low value during the curing cycle. After cooling, the specimen is removed from the press and it is found that the cords are firmly imbedded in the cured elastomer stock, but are visible on the surface. This sheet is cut into 1" wide strips, each having a cord in the center of its width. The cord end is separated from one end of the strip; the free end of the elastomer strip so obtained is clamped in the upper jaw of an Instron testing machine and the freed end of cord in the lower jaw. The machine is then operated to separate the jaws and thereby to strip the cord from the elastomer sheet in a continuous manner. The tension necessary to strip the cord from the elastomer sheet is determined and is reported in pounds tension per single end of cord. For determination of hot adhesion, the sample is brought to a temperature of 140 C. and held there while the cord is stripped from the elastomer sheet.

The RFL adhesive mixture of Examples I and II is prepared by mixing 1.38 parts by weight of resorcinol, 2.02 parts of 37% formaldehyde, 2.39 parts of 8.5% aqueous sodium hydroxide, and 27.8 parts water. This mixture is aged for six hours at 75-78 F. and then added to a mixture of 30.5 parts of 41% Solids butadiene/styrene/vinylpyridine (in a mole ratio of 70/ 15/ 15) terpolymer latex diluted with 7.64 parts water. The final mixture is allowed to age for 12 hours before using. After applying it to the shaped structure, the RFL mixture is cured at 190 C. to 245 C., preferably 205 C. to 212 C., for 30-90 seconds with an applied stretch of 2% to +5 Usually the dry solids pickup in this step in tire cord treatment ranges from about 2% to 7% by Weight based upon the weight of the original cord.

The treated polyester structure may be bonded to rubber stocks of various types such as vulcanizable natural and synthetic rubbers including the butadiene polymers and copolymers with styrene, acrylonitrile, the vinyl pyridines and the like, as well as polymers of chloroprene and isoprene and combinations of two or more of such materials. In commercial applications the rubber stocks usually include reinforcing agents, pigments, vulcanizing materials, accelerators, antioxidants, and the like.

In the following examples, which illustrate specific embodiments within the scope of the invention, parts and percentages of materials are by weight.

Example I An adhesive mixture is prepared by dissolving in parts by weight of acetone, 2 parts of an epoxylated novolak resin having a softening point of 99 C., an average molecule weight of 1,270, an oxirane oxygen content of 6.67.0% by weight and an epoxide equivalent of 235. To the solution is added 1 part of a liquid polyamide resin prepared from linoleic acid dimer and a polyalkylene polyamine. The polyamide resin has an amine value of 290 to 320 milligrams per gram (milligrams of KOH equivalent to base content of 1 gram of polyamide as determined by titration with HCl).

A polyethylene terephthalate cord of 840 denier (93 tex), 2-ply construction, is dipped in the above-prepared adhesive mixture, dried at 93 C. for one minute, and then run through an oven at 220 C. under an applied stretch of 2% with an exposure time of one minute. The treated cord is then dipped in the RFL adhesive mixture described previously and again heated for one minute at 220 C. while undergoing an applied stretch of 2%. The total weight of adhesive coating on the cord, determined by weight increase during processing, is 4% based upon weight of greige cord.

The treated cord structure is found to be stiff and remarkably coherent. Cutting the cord diagonally does not product loose or frayed ends. Such a structure is considered ideal for the preparation of raw-edged V-belts.

The experiment is repeated using trichloroethylene as the solvent instead of acetone to give a second test sample.

Adhesive-treated cords prepared above are tested for adhesion in the single-end strip adhesion test using both a neoprene rubber stock and a natural rubber stock having the following composition:

The adhesion test results are summarized in the following table.

In the same SESA test, cords treated only with the RFL adhesive mixture give test values of less than 1.0 pound.

Example II An adhesive mixture is prepared by dissolving in 96 parts acetone, 4.4 parts of the epoxylated novolak described in Example I, and adding 4.4 parts of the liquid polyamide resin described in Example I dissolved in 96 parts methanol. The liquid mixture is then applied to a fabric woven from polyethylene terephthalate fibers, dried and then cured for one minute at 200 C. to give a total solids pickup of 2% by weight, based on weight of coated fabric.

The treated fabric is then further coated with the RFL mixture described previously, and cured for 2 minutes at 200 C. to give a total solids pickup in both layers of 11% by weight. The treated fabric is bonded to a sheet of natural rubber of the type described in Example I by heating under pressure at 157 C. for 30 minutes, and cut into oneinch-wide strips for testing. The test consists of measuring the tension developed during the stripping of fabric from rubber. The test is carried out at 145 C. and an average tension of 59 pounds is measured during the stripping operation.

In control tests carried out in a similar manner but modified by the use of the novolak resin alone or the polyamide resin alone as a first coating mixture, rather than a combination of the two, it is found that fabric strip adhesion values at 145 C. are about 3 pounds per inch of width for the novolak resin and no higher than about pounds per inch of Width for the polyamide resin.

Example III This example illustrates the preparation of a raw-edged V-belt from cords coated with the adhesive of this invention.

An adhesive mixture is prepared by mixing 3 parts of the epoxylated novolak described in Example I, 1.5 parts of the liquid polyamide resin described in Example I, and 95.5 parts of toluene. A polyethylene terephthalate V-belt cord of 1100/2/5 construction is dipped in the toluene solution and heated to 94 C. to evaporate the solvent, leaving a dry solids coating on the cord of 1.5% by weight. The treated cord is then hot-stretched in an oven at 218 C. while undergoing a 3% applied stretch. Exposure time is 90 seconds. This cord is then further dipped in an RFL mixture prepared by adding a mixture of 7.37 parts of resorcinol, 4.0 parts of 37% formaldehyde, and 14.8 parts of water which has been aged for 1 hour at 24 C., to 24.0 parts of 41% solids butadiene/ styrene/vinyl pyridine (70/15/15) terpolymer latex diluted with 23.9 parts water, and the final RFL mixture is aged for 4 days before using. After dipping in the RFL mixture the cord is heated at 232 C. for 90 seconds while maintaining its length constant. Dry RFL solids pickup is 2.5% of the weight of the original cord. The cord has become a stiff, coherent structure.

The adhesive-treated cord is tested for adhesion to rubber in the single-end strip adhesion test using a neoprene rubber stock and found to give an SESA value of 30 pounds at room temperature. Failure is by tearing of the rubber rather than failure of the adhesive interface.

Following well-known techniques for preparing rawedged V-belts, a cover fabric is wound on a collapsible, cylindrical building drum and covered with a sheet of neoprene bottom rubber calendered to the proper thickness. A thin sheet of neoprene cushion rubber is then Wound on the drum to cover the bottom rubber. Next the building drum is rotated and the adhesive-coated cord prepared above is wound on top of the cushion rubber in a spiral fashion by feeding it from a supply head traveling parallel to the axis of the drum. A single layer of closely-spaced side-by-side cords is applied, and this is then covered with a second layer of neoprene cushion rubber which is applied and rolled down with a roller. Finally, the V-belt top-cover fabric is applied. The drum is then removed from the building machine and placed in an autoclave to cure the belt assembly. After curing, the belt assembly is removed from the building drum and placed on a mandrel for cutting. By rotating the mandrel against a pair of stationary knife blades, the 2-foot-wide belt assembly is cut into individual raw-edged V-belts having a top Width of We inch. The resulting structure is illustrated in FIGURE 2.

V-belts prepared as above are tested on a 3-pulley dynamometer stand at 4900 r.p.m. with a load of 70 lbs. on the idler pulley. The belts show no cord fraying along the cut edge, and no loss of power transmission due to cord fraying can be observed. The belt fails by elastomer and cord fatigue rather than by adhesion failure.

The process of the invention may be applied to the treatment of any fibrous material useful in the reinforc ing of rubber products, such as cotton, rayon, nylon, and the like. However, especially valuable results are obtained when the invention is applied to polyester fibers, cords, and other shaped structures such as those prepared from polyethylene terephthalate, because of the inadequacy of previously known adhesive systems. Illustrative of the polyesters useful in preparing fibers, films or other shaped structures which may be bonded to rubber by the process of this invention are those disclosed in US. Patents No. 2,465,319, No. 2,965,613, and No. 2,901,466.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations except to the extent defined in the following claims.

I claim:

1. In the production of raw-edged V-belts which includes building a wide rubber belt having internal reinforcing filamentary polyester cords bonded to the rubber and then cutting the wide belt into a plurality of narrower belts whereby longitudinal sections of the cords are exposed, the improvement in the bonding of polyethylene terephthalate filamentary cords to the rubber to provide raw-edged V-belts reinforced with coherent polyester cords highly resistant to fraying at the raw edges; wherein the improvement comprises 1) coating the cords with a volatile organic solvent solution containing dissolved therein 0.5% to 20%, by weight of the solution, of

(A) an epoxylated novolak resin that is the reaction product of a non-heat-hardenable phenolformaldehyde condensation product with epichlorohydrin having an average of at least 2 epoxy groups in each molecule, an average molecular weight above about 540 and an epoxide equivalent within the range of 200 to 300, and (B) a polyamide resin reaction product of polymeric fat acids having at least two carboxyl groups and polyalkylene polyamines having two primary amine groups and a plurality of secondary amine groups, the polyamide resin being liquid and having an amine value of at least 200, and wherein the weight ratio of (A)/ (B) is from /20 to 20/80; (2) heating the coated cords to a temperature of 7 C. to 240 C. to evaporate the organic solvent and cure the coating;

(3) overcoating the cords with an aqueous resorcinolaldehyde and butadiene-vinyl pyridine latex mixture;

(4) drying and curing the overcoating; and

(5) applying and curing a layer of rubber on the coated cords.

2. The process as defined in claim 1 wherein the epoxylated novolak resin (A) has a molecular weight of 1100 to 1500, an oxirane oxygen content of 5% to 10% by weight, and an epoxide equivalent of 210 to 260.

3. The process as defined in claim 1 wherein the polyamide resin (B) has an amine value of 290 to 320.

4. The process as defined in claim 1 wherein the coating produced by steps (1) and (2) constitutes from about 0.5% to 20% by weight of the coated cord.

5. The process as defined in claim 1 wherein the overcoating applied to steps (3) and (4) is an aqueous recorcinol-formaldehyde and butadiene/styrene/vinylpyridine (70/15/15) terpolymer latex and the resulting overcoating is about 2% to 7% of the weight of the original cord.

References Cited UNITED STATES PATENTS 2,773,540 12/1956 Waugh 156-142 X 2,902,398 9/ 1959 Schroeder 156-315 2,909,204 10/ 1959 Somerville 161-184 2,962,468 11/ 1960 Groves 260-42 3,036,948 5/1962 Danielson.

3,136,681 6/1964 Johnston 161-190 3,190,764 6/1965 Cardina 117-62.2 3,222,238 12/1965 Krysiak 156-330 3,247,043 4/1966 Cardina 156-314 

